Material for modifying semiconductors



United States Patent 3,151,386 MATERHAL FOR MODIFYING SEMICONDUCTORS Clyde E. Ingersoil, Tonawanda, N.Y., assignor to Williams Gold Refining (30., Inc., Buffalo, N.Y. N0 Drawing. Filed Mar. 22, 1962, Ser. No. 181,816 5 Claims. (Cl. 29-182) This invention relates to material for use in modifying semiconductors and to the method of making such material.

In the manufacture of semiconductors, it is desirable to use a non-contaminating carrier for the metalloid to be diffused into the semiconductor body; tohave the metalloid uniformly distributed throughout the carrier to enhance the diffusion process; and to establish uniformity in the percentage of metalloid in the carrier so that the diffusion process can be assured of continuing efliciency. As far as is known, only the first of these desiderata has been obtained, the achievement of uniform distribution of the metalloid and precise control as to percentage of metalloid having been largely unobtainable. Moreover, known prior art efforts have been characterized by a considerable degree of expense due mainly to vaporization loss of metalloid.

It is therefore a primary object of this invention to provide material in foil, sheet or strip form which comprises a mechanical mixture of a carrier metal or metals and a metalloid useful in modifying semiconductors. In par ticular, this invention contemplates gold, silver and mix tures of gold and silver as the carrier and utilizes the metalloid boron in combination therewith. By providing a mechanical mixture, both uniform dispersion and accurate control of the proportion of metalloid-to-carrier can be achieved.

Another object of this invention resides in the provision of a mechanical mixture of a non-contaminating carrier and boron in which the boron is advantageously dispersed within the carrier so that the material is expeditiously used for modifying semiconductors.

More specifically, an object of this invention is to provide a material of foil or similar form which contains a uniform dispersion of discrete particles of boron for subsequent diifusion within a semiconductor, the carrier for the boron particles being of such character as to obviate contamination of the semiconductor during the diffusion process while at the same time being of suflicient malleability as to be successfully rolled .into foil form.

A further object of the invention is to provide a material as aforesaid wherein the mechanical mixture of the carrier and the boron is of an actual density closely ap proximating its theoretical density so as to enable economic and efficient treatment of semiconductors.

Another object of this invention is to provide a method for making material as set forth above in which a mechanical mixture of a carrier and boron is first cold pressed to render the mixture easily handled and then consolidated by heat and/or pressure to obtain an actual density thereof suificiently high as to permit the material to be rolled to foil form.

More specifically, the aboveobject contemplates cold pressing followed by sintering to yield material having an actual density of at least about 50% of theoretical density. Still more specifically, whereas the actual density after sintering may be sufficient to permit cold rolling to be performed without further actual density increase by hot pressing, it is often preferred to increase the sintered actual density by hot pressing so that the cold rolling process may progress more easily and without the necessity for delicate control thereof.

Another object of the invention is to provide an im-.

3,151,386 Patented Oct. 6, 1964 "Ice Another object is to provide an improved method as aforesaid which involves only simple and easily practiced shop techniques.

Other objects and advantages of the invention will appear from the specification hereinafter.

Broadly stated, the invention relates to a material, and method for making the same, useful for modifying semiconductors. For example, when it is desired to diffuse semiconductors of the germanium and silver types with boron, a strip or foil comprising a mechanical mixture of boron and gold in accord with the present invention may be employed. Thus, the strip or foil of the invention comprising the desired boron content'carried within a matrix of a suitable carrier metal such as gold, silver or mixtures thereof, may then be applied to the semiconductors under such conditions that diffusion will take place.

More specifically, the strip or foil may be prepared by thoroughly mixing appropriate proportions of carrier metal and appropriately sized boron particles. This mixture is then subjected to suitable compacting whereby to provide a structurally integrated bar comprising a mechanical mixture of gold and boron having an actual density in the order of at least about 50% of its theoretical density. This bar is then cold rolled into strip or foil form and then out into appropriate stock sizes for convenience of handling when applying it to the semiconductor materials to be treated. The cold rolling is performed in stages with intermediate annealing and in order to enhance this process, a greater actual density than as above specified may be necessary. However, this will depend upon the nature of the carrier metal and the amount of boron used. Generally, the higher the boron content, the greater is the actual density required after compaction and, in general, the use of silver or high silver content carrier permits of cold rolling at lower initial actual densities.

tration.

The selection of the matrix material for the composite strip or foil of the invention is dependent upon the "characteristics of the semiconductor being treated and the requisite finished characteristics thereof. For example, to avoid certain contaminants a matrix of substantially pure gold will in many cases provide the required performance. In other cases, the matrix may be of pure silver, and will provide the finished product with the requisite metallurgical content. In other instances,: mixtures or alloys of gold and silver may be used advantageously. To provide those-skilled in the art with a more complete understanding of the invention, the following illustrative examplesare given.

Example 1 in the proximate proportions er 99 partsi (by.iweight) of gold to one part of boron; "This mixture is then subjected to coldpressing at about 3 tons per square inch, -to .give the mixture sufiicient, body to be handled. The partially compacted mixture is then sintered at a temperature of 1850 for 15 to 20 minutes yielding further compaction of the material to'an actual density of about 65% theoretical density. After sintering, the material is subjected to final compaction to increase its actual density to at least about 75% and preferably about 85% of its theoretical density. This final compaction is achieved by hot pressing, as for example, with a pressure of 7.5 tons per square inch and a temperature of 1700 F. Several hot pressing steps will normally be required to obtain the requisite actual density. The material is then rolled to final thickness which, in this case, is .001 inch.

The technique of the rolling process is dependent upon the actual density obtained after hot pressing. That is, the initial 50% reduction of the material is brought about by differing techniques dependent upon the actual density after hot pressing. For example, with an actual density after hot pressing in the neighborhood of 75%, the initial 50% reduction is carried out in steps of about with annealing after each step. With an actual density of 85% or higher, the initial 50% reduction may be carried out in steps of about with annealing after each step. For densities in between, generally proportional reductions may be made. In any case, once about 50% initial reduction is achieved, the material may be brought to about .010 inch thickness in steps of about .020 inch each, with annealing after each step, whereafter a reduction to final size Without annealing (.0005 to .002 inch thickness) may be made.

Example ll Highly purity silver powder is mixed with powdered boron in the ratio of 99.5 parts (by weight) of silver to 0.5 part boron. In order to structurallf integrate the mixture, it is first placed in a mold and subjected to a cold pressing operation at a pressure of about 3 tons per square inch, achieving an actual density of about 70% of theoretical density and a thickness of about .091 inch. After such integration, the material is removed from the mold and sintered at a temperature of 1700 F. for minutes whereby further compaction to .087 inch thickness and actual density of about 79% occurs. The material is then subjected to hot pressing at 1600 F. and about 7.5 tons per square inch to render a thickness of .052 inch and an actual density of about 90% of theoretical density. The material is then rolled in steps of about 20% reduction to a thickness of about .030 inch, with annealing after each step, whereafter it is rolled to .010 inch thickness, annealed, and then rolled to final size of .001 inch thickness.

Although this example can dispense with the hot pressing compaction, it is desirable to utilize the same since the high degree of actual density obtained considerably simplifies the cold rolling process.

Example III Sintered material in accord with Example II was rolled to size without the hot pressing process. The quality of the product was the same as in Example 11 although the rolling technique, as set forth generally in Example I, was required. 7

Example IV Several comparative examples having various gold and silver content were prepared by the method of Example II, that is, with cold pressing, sintering and hot pressing preceding the rolling operations, as follows:

Percent Subsequent rolling was performed in accord with the practice set forth in Example I, the boron content in each case being about 1%.

Example V Gold and boron as in Example I were mixed to render a mixture containing boron. This mixture was compacted by first cold pressing with pressure of about 3 tons per square inch and sintering at 1850 F. for 15-20 minutes, yielding an actual density of about 75% of theoretical density. With the small amount of boron involved, the sintered density is high enough to permit the hot pressing step to be eliminated. The thus compacted material was then cold rolled to final size.

Example VI The procedure of Example V was followed except for using silver as the carrier, similar to Example II, and the sintering temperature was 1700 F. Again, as in Example V, no hot pressing was required.

Example VII Examples V and VI were repeated using 0.01% boron.

From the above examples, it will appear that the actual density subsequent to sintering is of importance in determining subsequent steps. Material having an actual density of at least about of thoretical density can be rolled directly without hot pressing. However, in general, the higher the actual density prior to rolling, the fewer number of rolling passes and annealing steps are required to obtain the same final size. The cold pressing prior to sintering is utilized primarily to obtain structural integrity of the material sufficient to permit its re moval from the mold for introduction into the sintering furnace.

Whereas no problem is encountered in introducing very small quantities of the metalloid, very great difficulty in cold rolling, regardless of the actual density after compaction, is encountered with a metalloid content materially greater than 1%. No sharp line of demarcation was, however, observed since variations between different batches of raw material, carrier and metalloid, appeared to indicate a somewhat variable upper 'limit of metalloid. However, it can be safely stated that about 3% metalloid, by weight and regardless of compacted actual density, represents an upper limit insofar as this invention is concerned, even under the most favorable conditions of starting materials. Likewise, it may be stated that under any conditions of starting materials, 2% metalloid represents the practical upper limit, regardless of compacted actual density.

It is to be understood that certain changes and modifications as illustrated'and described may be made without departing from the spirit of the invention or the scope of the following claims.

Iclaim:

1. A material for use in the manufacture of semiconductors, comprising foil having a thickness in the order of .0005 to .002

inch,

the foil consisting of a mechanical mixture of boron and a metal selected from the group consisting of (a) silver, (b) a mixture of silver and gold, and gold,

in which the boron is present in a small amount of not more than about 3% by'weight.

2. A material for use in the manufacture of semiconductors, comprising foil having a thickness in the order of .0005 to .002

inch,

the foil being a mechanical mixture of a carrier and boron,

the mixture having an actual density approximating its theoretical density, v

the boron being present as discrete particles substantially uniformly dispersed throughout the foil,

said carrier being selected from the group consisting of gold, silver and a'mixture of gold and silver,

said boron being present in a small amount of not more than about 3% by weight of the mixture. 3. A material for use in the manufacture of semiconductors, comprising foil having a thickness in the order of .0005 to .002 inch consisting of a mechanical mixture of a carrier and boron in which the boron is in the form of discrete particles in a minor amount of not more than about 3% by weight of the mixture, said carrier being metal selected from the group consisting of gold, silver and a mixture of gold and silver. 4. A material for use in the manufacture of semi conductors, comprising foil having a thickness in the order of .0005 to .002

inch, the foil being a mechanical mixture of a carrier and boron having an actual density approximating its theoretical density,

dispersed throughout the mixture,

said carrier being selected from the group consisting of gold, silver and a mixture of gold and silver, said boron being present in a minor amount of not more than about 3% by weight of the mixture. 5. A material according to claim 2 wherein the amount of boron does not materially exceed 1% by weight of the mixture.

' References Cited in the file of this patent UNITED STATES PATENTS 2,221,286 Hensel NOV. 12, 1940 3,060,018 Desmond Oct. 23, 1962 3,068,127 Patalong et al. Dec. 11, 1962 FOREIGN PATENTS 782,688 Great Britain Sept. 11, 1957 799,946 Great Britain Aug.'13, 1958 OTHER REFERENCES Iron and Steel Engineer, July 1959, vol. XXXVI, No. VII, pp. 118424. 

1. A MATERIAL FOR USE IN THE MANUFACTURE OF SEMICONDUCTORS, COMPRISING FOIL HAVING A THICKNESS IN THE ORDER OF .0005 TO .002 INCH, THE FOIL CONSISTING OF A MECHANICAL MIXTURE OF BORON AND A METAL SELECTED FROM THE GROUP CONSISTING OF (A) SILVER, (B) A MIXTURE OF SILVER AND GOLD, AND (C) GOLD, IN WHICH THE BORON IS PRESENT IN A SMALL AMOUNT OF NOT MORE THAN ABOUT 3% BY WEIGHT. 